Seatbelt restraint systems for restraining an occupant in a vehicle seat play an important role in reducing occupant injury in vehicle crash situations. Seatbelt restraint systems of the conventional so-called “3-point” variety commonly have a lap belt section extending across the seat occupant's pelvis and a shoulder belt section crossing the upper torso, which are fastened together or are formed by a continuous length of seatbelt webbing. The lap and shoulder belt sections are connected to the vehicle structure by anchorages. A belt retractor is typically provided to store belt webbing and may further act to manage belt tension loads in a crash situation.
Seatbelt restraint systems which are manually deployed by the occupant (so-called “active” types) also typically include a buckle attached to the vehicle body structure by an anchorage. A latch plate attached to the belt webbing is received by the buckle to allow the belt system to be fastened for enabling restraint, and unfastened to allow entrance and egress from the vehicle. Seatbelt systems, when deployed, effectively restrain the occupant during a collision.
Some seatbelt restraint systems include pretensioning devices, which tension the seatbelt either prior to impact of the vehicle (also known as a “pre-pretensioner”) or at an early stage of a sensed impact to enhance occupant restraint performance. The pretensioner takes out slack in the webbing and permits the belt restraint system to couple with the occupant early in the crash sequence. Upon the detection of a condition leading to an imminent impact or rollover, or in the event of an actual rollover, seat belt webbing is automatically and forcibly retracted by the pretensioner to tighten the seat belt against the occupant.
One type of pretensioning device is a pyrotechnic linear pretensioner (PLP), which can be implemented as a pyrotechnic buckle pretensioner (PBP) that is attached to a seat belt buckle. PLPs can also be attached to a webbing guide loop or seatbelt anchorage. Since both types pull a seat belt system component linearly to apply tension in the belt webbing, both PLPs and PBPs can be collectively referred to as PLPs. Examples of designs of PLPs and PBPs are provided by U.S. Pat. Nos. 6,068,664 and 7,823,924, which are hereby incorporated by reference. Typical PLPs have a pyrotechnic charge that is fired when a collision occurs, producing expanding gas which pressurizes a gas chamber within a tube, which forces a piston down the tube. The piston is connected with the belt system by a cable or strap. Stroking of the piston tightens or “pretensions” the belt against the occupant.
One limitation of known PLP designs in which the piston stroke shortens a cable attached to one end of seatbelt webbing is that this configuration limits the PLP mounting location to the side of the seatbelt buckle because the other end of the seatbelt is rolled up on a spool.
Also, in order to retract the cable, the cable is connected to the piston and is pulled into the expanding gas chamber. Sealing the gas chamber around the flexible cable presents a difficult challenge. Pulling the cable past the seal generates friction that needs to be compensated with a greater pull force. Also, the location at which the cable enters the gas chamber creates a potential pressure leak path. The gas escaping from the device into the vehicle passenger compartment may lead to reduced efficiency of the pretensioner and may contain combustion products that may negatively affect seat occupants.
These shortcomings are typically compensated by using gas generators producing a greater gas volume to ensure a piston stroke sufficient to remove the slack in the seat belt. The gas chamber needs to be long enough for the piston to move by the distance that corresponds to the slack in the seat belt. All these properties of existing linear pretensioner devices make PLPs rather large and complex.